Yesterday I received my headphones.
I think "EXTRA BASS'" has only become a trade mark of sony and nothing more than that. Currently I am using a 5 year old sony MDR-EX150(without "EXTRA BASS") which is around 5 times less expensive than SP-510, but has nearly equal sound quality.
Sony MDR-EX255(without "EXTRA BASS") also uses a 12mm driver, but has got more punch than SP-510.
The bass can be at best described as in Sony NWE393 Walkman after switching off the "CLEAR BASS".
Clarity is good, but you can get this type of sound quality in less expensive headphones also.
Not happy after the purchase.